1. Field of the Invention
The present invention relates to a piezoelectric actuator wherein piezoelectric elements are used as a drive source. This invention also relates to a piezoelectric pump injector incorporating such an actuator.
2. Description of the Prior Art
Already known in the art is the use of piezoelectric elements as a drive source of a high speed actuator designed to operate fuel injectors or control fuel injection systems of an internal combustion engine. As piezoelectric elements develop stress almost instantaneously upon application of an electric field, the use of a piezoelectric actuator enables the control of fuel injectors and fuel injection systems to be carried out with a more precise timing when compared with a mechanical actuator. Furthermore, a piezoelectric actuator may be electronically controlled by a computorized control unit so that the quantity of fuel injected may be controlled with a wider flexibility in dependence with various operating conditions of the engine.
In its simplest form, a piezoelectric actuator includes a hollow casing in which is mounted a column of piezoelectric elements. The column of piezoelectric elements generally comprises a plurality of thin piezoelectric disks of piezoelectric material numbering, for example, one hundred, and stacked one upon another with interleaved foil electrodes sandwiched therebetween. Each electrode is connected alternately to a plus lead line and a minus lead line. When an electric voltage of, for example, about 500 V, is applied between the plus and minus electrodes to produce an electric field across each piezoelectric disk, the latter expands in the direction of the thickness, thus displacing the free end of the column of piezoelectric elements in the axial direction. The axial displacement of the column end is taken to the outside of the actuator by means of an output mechanism such as an output rod or piston.
One of the problems of the prior art piezoelectric actuator is that breakdown of the insulation of the piezoelectric disks occurs when foreign materials such as moisture and electrically conductive particles are deposited on the periphery of the foil electrodes and piezoelectric disks. Moreover, the piezoelectric property of the piezoelectric material may be degraded if the temperature of the piezoelectric material becomes higher than the Curie point, due to a build-up of heat resulting from repeated high frequency energization of the piezoelectric element.